1. Field of the Invention
The present invention relates to a zoom lens capable of selectively setting zooming to a desired focal length within a range of a predetermined focal length, and particularly, to a zoom lens aimed to achieve widening of a field angle of a wide-angle end and high magnification power in addition to miniaturization and improvement in performance, which is suitable for a camera utilizing an electrically imaging device such as a digital camera and a video camera, and to a lens unit, a camera and a portable information terminal device using such a zoom lens.
2. Description of the Related Art
A camera of a type that images an image of a subject to be photographed with a solid-state imaging device such as a CCD (Charge-Coupled Device) imaging device or the like, acquires image data of a static image (still image) or a motion image (movie image) of the subject thereby, and digitally stores the acquired image data into a nonvolatile semiconductor memory or the like represented by a flash memory, which is referred to as a digital camera or an electronic camera etc., has been generalized already. Such a type of camera is rapidly replacing a conventional type of camera utilizing a traditional silver salt film, more specifically, the silver salt camera.
A market for such a digital camera has been a very large one, and the user's demands for the digital camera have been ranged in scope. In particular, improvement in image quality and miniaturization of the digital camera are always the user's demand, and are of large interest of the user. Therefore, compatibility of improvement in performance and miniaturization is requested for a zoom lens used as a photographing lens as well.
In order to accomplish the miniaturization, it is necessary to shorten an overall length of lenses when in use, in other words, to shorten a distance from a lens surface which is nearest to an object side to an image plane, and also, it is important to reduce thickness of each group of lenses to curb the overall length at the time of storage. In addition, to accomplish the improvement in performance, it is necessary to secure resolving power corresponding to an imaging device having the number of pixels of at least 3 million to 5 million pixels or more throughout the entire zooming range.
Additionally, there are number of users who desire widening of a field angle of the photographing lens, so that it is desirable to provide a half field angle of a wide-angle end of the zoom lens to be more than 38 degrees. 38 degrees of the half field angle is equivalent to a focal length of 28 mm in terms of a 35 mm silver salt camera using a 35 mm size (so-called Leica size) silver salt film.
Furthermore, it has been desired that magnification power of zooming is as large as possible. It is considered to be possible to deal with most of the general photographing if a zoom lens has a focal length of worth 28 mm to 135 mm in terms of the 35 mm silver salt camera, and the magnification power of such a zoom lens is approximately 4.8 times. Accordingly, equivalent or higher magnification power with respect to such a zoom lens is desired in the digital camera as well.
Meanwhile, there are many types of zoom lenses which can be considered for the zoom lens to be used in the digital camera. A zoom lens having more than four groups of lenses has a tendency that total thickness of the entire lens systems becomes large, and consequently, there is a limit in shortening of the overall length of lenses and is not suitable for miniaturization, accordingly. Also, as a most general type of zoom lenses having magnification power of 3 times, there is a zoom lens comprising a first group of lenses having a negative focal length, a second group of lenses having a positive focal length and a third group of lenses having a positive focal length which are subsequently arranged from an object side to an image side, and having an aperture stop provided at the object side of the second group of the lenses that moves integrally with the second group of the lenses, and in accordance with changes in magnification power carried out from a short focal end to a long focal end, the second group of the lenses monotonously moves from the image side to the object side, and the first group of the lenses moves so as to correct fluctuation of an image plane position caused in accordance with the magnification power.
However, the zoom lens of such a structure is not suitable for attaining high magnification power exceeding magnification power of 4 times.
Within the zoom lenses of such a structure mentioned above, there is a type of zoom lenses, in which the number of group of lenses is three, which is less, and which is suitable for attaining relatively high magnification power, comprising a first group of lenses having a positive focal length, a second group of lenses having a negative focal length and a third group of lenses having a positive focal length which are subsequently arranged from an object side to an image side, and having an aperture stop provided between the second group of the lenses and the third group of the lenses, and in accordance with changes in magnification power carried out from a wide-angle end to a telephoto end, at least the first group of the lenses and the third group of the lenses are moved such that an interval between the first group of the lenses and the second group of the lenses becomes gradually larger and an interval between the second group of the lenses and the third group of the lenses becomes gradually smaller. The zoom lenses of such a type is, for reference, disclosed in JP-A H11-109236, JP-A H11-142733, JP-A H11-242157, etc.
More specifically, the zoom lens disclosed in JP-A H11-109236 has large magnification power of 10 times, but the half field angle in the wide-angle end is obtained only to the extent of 31 degrees, which is totally insufficient in terms of the widening of the field angle. Also, JP-A H11-142733 discloses an example of the zoom lens having the magnification power of 3 times to 5 times and in which the half field angle in the wide-angle end is 25 degrees to 34 degrees. However, according to the detailed example thereof, the variable ratio of only 3 times is maintained in an example in which a zoom lens having a relatively wide half field angle of 34 degrees in the wide-angle end is shown, which cannot be said that compatibility of the widening of the field angle and the achievement of the high magnification power is sufficient. The zoom lens disclosed in JP-A H11-242157 has the magnification power of 3 times to 6 times and is relatively compact in size, but similar to the case in JP-A H11-109236, the half field angle in the wide-angle end is maintained only to the extent of 31 degrees, which is, again, insufficient in terms of the widening of the field angle.